Implantable medical devices (IMDs) are devices designed to be implanted into a patient or subject. Some examples of these devices include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization devices, and devices that include a combination of such capabilities. The devices are typically used to treat patients using electrical therapy or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include electrodes in communication with a sense amplifier to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. Other examples of implantable medical devices include implantable diagnostic devices, implantable insulin pumps, devices implanted to administer drugs to a patient, or implantable devices with neural stimulation capability.
Heart sounds are associated with mechanical vibrations from activity of a patient's heart and the flow of blood through the heart. Heart sounds recur with each cardiac cycle and are separated and classified according to the activity associated with the vibration. The first heart sound (S1) can be thought of as the vibrational sound made by the heart during tensing of the mitral valve. The second heart sound (S2) can be thought of as marking the beginning of diastole. The third heart sound (S3) and fourth heart sound (S4) can be conceptualized as related to filling pressures of the left ventricle during diastole. Heart sounds are useful indications of proper or improper functioning of a patient's heart. The present inventors have recognized a need for improved sensing of events related to cardiac activity.